1. Field of the Invention.
The field of the invention relates to the manufacture of shoes wherein one molded piece may be used in conjunction with shoes of several different sizes.
2. Brief description of the prior art.
Unit soles have been used to advantage in the manufacture of footwear for a number of years. An example of such a sole is provided in U.S. Pat. No. 2,995,840. In this patent, the sole is molded as an integral unit comprising a forepart, shank, and heel. An upper and insole are secured to the sole to arrive at a finished shoe.
The present system for making molded unit soles requires a right and left mold for each size of shoe to be made. A typical men's size run would be: 7-71/2-8-81/2-9-91/2-10-101/2-11-12-13, or 11 pair. Typical women's sizes are: 5-51/2-6-61/2-7-71/2-8-81/2-9-91/2-10. The fit between the unit sole and the lasted upper is critical, and a particular unit sole can only be used with a last made to go with it. The pattern of the bottom of the last, plus an allowance for the upper leather, box toe, counter, etc., must fit very close to the raised edge of a dress unit sole, to give an acceptable appearance. This requirement puts an investment in unit sole molds at risk, because if style changes move away from the toe shape or other characteristics of a particular last, the unit soles that go with it can go "out of style" before the investment is amortized. Present costs are substantial even for plastic molds suitable for short runs of low pressure urethane foam molding, and higher for aluminum molds for vinyl materials that are injected at medium pressure. Steel molds for difficult materials are more expensive.
These costs and the style risk have limited unit soles in several ways. In medium and high grade men's shoes, the normal run is 6 to 14, not 7 to 13. Also, these shoes are usually sold in several widths, like B C D E and EEE. It is possible to combine adjacent widths onto one last bottom pattern, so that two sets of molds might cover B-C-D and E-EEE, but that is still more than double the mold cost for a limited size run in a single width. The style obsolescense risk is also greater in higher grade shoes, because it takes longer in those grades to sell enough pairs of an item to pay for the set of molds.